In recent years, a semiconductor device such as light-emitting device (e.g., semiconductor light-emitting element, light-emitting diode, semiconductor laser) and electronic device is increasingly demanded to realize high density and high resolution of optical recording or the like and for this purpose, a nitride semiconductor capable of emitting blue light is attracting attention. In particular, it is required to provide a nitride semiconductor substrate with less crystal defects and good crystallinity.
In order to grow such a good nitride semiconductor on an original substrate, the original substrate selected and used is preferably lattice-matched to the objective nitride semiconductor. However, in fact, an original substrate lattice-matching to the nitride semiconductor is difficult to obtain and at present, a nitride semiconductor is grown on an original substrate which is a dissimilar material to the nitride semiconductor but has sufficiently high heat resistance or chemical stability. For example, various techniques of growing a nitride semiconductor by using a relatively inexpensive sapphire or zinc oxide original substrate have been heretofore proposed.
For example, JP-A-7-165498 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A-7-202265 disclose a method of forming a buffer layer comprising zinc oxide on a sapphire original substrate, growing a nitride semiconductor thereon, and removing the buffer layer to obtain a nitride semiconductor. However, this method has a problem that since the buffer layer formed on a sapphire original substrate has bad quality, the nitride semiconductor formed thereon comes to have crystal defects as many as 108/cm2 or more and a good-quality nitride semiconductor is not obtained.
In order to overcome such a problem, JP-A-11-251253 proposes a method of forming an underlying layer on a sapphire original substrate, then forming a mask layer comprising SiO2 or the like, thereby artificially applying a surface processing, and further growing thereon a nitride semiconductor. According to this method, a nitride semiconductor having less crystal defects as compared with the case of growing a nitride semiconductor on a buffer layer comprising zinc oxide can be obtained. However, the nitride semiconductor obtained by this method has a problem that a pattern attributable to the artificial surface processing applied to the original substrate is formed therein. For example, when a striped surface processing is applied to the original substrate, a striped pattern is formed in the nitride semiconductor grown thereon. Therefore, a portion with good crystallinity must be selected and used out of the produced nitride semiconductor and this is inconvenient.